This invention relates to a process for the preparation of microporous films of ultrahigh molecular weight polyethylene by forming a solution thereof into a film and removing the solvent from the film, as well as to films that can be obtained in this way.
In the context of this invention the expression "(micro)porous film" is understood to mean not only a film in the ordinary, general sense, but also a tube, a tubular film, one or more hollow filaments with micropores.
Such a process is described in patent application WO-A-86/02282. The solvent should be a porogene, which according to said patent application should be understood to be a material that can be mixed with ultrahigh molecular weight polyethylene, for the sake of brevity denoted as UHMWPE, be heated to form a solution of porogene and polymer, and subsequently cooled to form a mixture comprising a separate phase that is rich in porogene. According to said patent application this phase can be isolated from the mixture and a microporous structure can be made of it. When a solution of UHMWPE in such a porogene is extruded to a film, phase separation is stated to occur in the film upon cooling. The porogene must then be removed from the film.
Though the above-mentioned patent application mentions aliphatic, alicyclic and aromatic hydrocarbons as possible solvents, in the examples use is made exclusively of a mineral oil having a viscosity of 60 cSt at 40.degree. C., i.e a dynamic viscosity of about 52 mPa.s at 40.degree. C. An oil having such a viscosity contains considerable amounts of high-boiling hydrocarbons, a large part of which cannot, or only with decomposition, be distilled at atmospheric pressure. The mineral oil can be removed from the film only by extraction, as also described in the examples of WO-A-86/02282. In doing so, the extractant needs to be refreshed at least once, and after extraction any residual extractant is to be removed from the film by evaporation. Such a process not only is laborious, time-consuming and expensive, but mineral oil extraction is not complete, either, and the residual amounts of mineral oil in the film are not small, which is undesirable.
The preparation of porous films of UHMWPE on the basis of UHMWPE solutions is described also in EP-A-160.551. Though it is stated there that the solvent may be any solvent capable of dissolving the polyethylene to a sufficient extent, for instance aliphatic or cyclic hydrocarbons, such as nonane, decane, undecane, dodecane, decalin and paraffin oils or mineral oil fractions with corresponding boiling points, but in the examples use is made exclusively of a liquid paraffin having a viscosity of 64 cSt at 40.degree. C. In this method, otherwise already complicated, the solvent, preferably a non-volatile solvent, must also be extracted from the film using a readily evaporating solvent, which has the disadvantages already outlined in the above.